- Vibrationally mediated photodissociation is emerging as a useful tool for studies of intramolecular dynamics in polyatomic molecules since it is a sensitive indicator of both the vibrational and electronic dynamics. The method involves energy deposition in skeletal motions of molecules, their subsequent promotion to excited electronic states and dissociation. Jet-cooled action spectra, reflecting the yield of the ensuing atomic fragments as a function of vibrational excitation, are compared to simultaneously measured room temperature photoacoustic spectra and simulated spectra. The narrowed action spectra expose resonance splitting and enable estimating the homogeneous broadening and thus time scales for intramolecular vibrational energy redistribution to close resonating states and to bath states. Overtone spectra of the acetylenic and alkyl C H stretches of the acetylene homologues, propyne and 1-butyne, demonstrate how the nature of rovibrationally excited states is highlighted by photolysis of pre-excited molecules.